Video discs systems are known in which a video program pre-recorded upon a disc, is played back through a television receiver by resort to a procedure which, in general, bears some similarity to the playback of audio records. While these known playback systems contemplate a variety of approaches, e.g., optical, capacitive, mechanical (pressure), etc., the present invention finds particular application to an optical system, accordingly, the invention will be described in that environment.
Optical image reproducing systems that permit playback of pre-recorded program material through a television receiver have been proposed to augment the utility of such receivers. In one system the program is stored in a disc, not unlike the familiar audio disc, which is interrogated by a beam of coherent light derived from a laser and converged by lens action to a finely focused reading spot. Subsequent to interrogation, the beam is monitored by a light responder, or photodetector, which serves to develop an electrical signal representative of the stored information.
The stored program can include luminance, chroma and audio information, as well as synchronizing components. This information is positioned in segments of the frequency spectrum which are convenient for disc recording, but at the same time, are readily subject to transformation in a transcoder to a frequency distribution typical of a commercial telecast. Since such telecasts feature two interlaced fields for each picture frame, the storage track of the video disc favors the form of a multi-turn spiral in which each convolution of the spiral contains two fields of an image frame together with synchronizing information.
Program information may be stored in the multi-turn spiral track of an optical video disc in a variety of ways, for example, in the form of hill and dale grooves similar to audio recordings, or, in a succession of pits alternating with lands. While the invention can be used with either type of recording, there presently appears to be greater interest in the pit and land arrangement, therefore, the invention will be described in connection with a storage track of that format.
In one approach, the pits and lands constituting the information storage track are of uniform width and length and the information is encoded as a rate variation of the train of pits with a nominal 50% duty cycle. In another encoding scheme, the width of the pits is maintained uniform but their length, along the track direction, is varied or modulated to encode the information. In either case, the pits and lands collectively comprise a spatial counterpart of an RF carrier signal, modulated in frequency or duty cycle. The modulated RF signal, in any event, conveys the program information and thus is employed in the recording process to control the fomration of a storage track on a disc master.
The stored information is retrieved by scanning the track with a light beam and utilizing a light responsive device, e.g., a photodetector, to monitor the beam subsequent to its interrogation of the track. In the case where the disc is transmissive to the reading beam, the photodetector can be positioned beneath the disc to collect a portion of the light transmitted therethrough. On the other hand, if the disc is a reflective device, the photodetector is located on the same side of the disc as the incident, or reading beam so as to respond to light reflected from the track. In either instance, the pits deflect or scatter the light of the reading beam causing the light responsive device to develop an electrical signal representative of the information stored in the disc.
Although the term scattering is commonly used to describe, in general, the effect of small pits or irregularities on a beam of incident light, we shall use this term in a more specific sense, that is, as it relates to a particular type of video disc, the so-called half wavelength, type. In this type of disc the depth of the pit is so chosen that there exists a one-half wavelength optical path difference between rays traversing a pit and the rays traversing the adjoining land area.
We shall use the term deflection in connection with the so-called one-quarter wavelength type of disc. In this type of disc, which is described in U.S. Pat. No. 3,931,459 and which issued to the present inventor and is assigned to the present assignee, the depth of the pit is chosen so that there exists a one-quarter wavelength optical path difference between rays traversing a pit and the rays traversing the adjoining land area.
Historically, the one-quarter wavelength system has been used with transmissive discs while the one-half wavelength system has found preference in connection with reflective discs. However, there are no compelling technical reasons for this and, in theory, transmissive and reflective discs are possible in either system. An optical video system having particular application to a transmissive type disc is described in U.S. Pat. No. 3,919,562 which issued to Robert L. Whitman on Nov. 11, 1975. On the other hand, an optical video system operating in the reflective mode is described in U.S. Pat. No. 3,959,581 which issued on May 25, 1976 to Leonard J. Laub.
In a known optical detection scheme, the information is retrieved by focusing the light of a laser beam to form a reading spot small enough to resolve the spatial frequency of the frequency modulated carrier. In this practice, severe requirements are imposed on the focusing apparatus to insure that any vertical excursions of the interrogated portion of the disc track are confined to a very narrow range since the depth of focus of an optical system capable of resolving the spatial frequency of the carrier is, of necessity, minute. Moreover, when the spatial counterpart of the carrier is optically detected and converted to an electrical signal representative of the modulated RF carrier, that signal must then be processed by a frequency disciminator in order to extract the information which was frequency modulated on the carrier.
In the subject teaching, consideration will be given to an optical detection system in which the reading spot is of a dimension that permits it to cover more than one track. Since adjacent tracks correspond to successive frames in the conventional 1800 RPM video disc format, reading adjacent track segments simultaneously can be tolerated because of the redundant nature of the recorded television program, i.e., one frame is much like the next, insofar as video content is concerned.
One advantage that is suggested in reading two or more tracks is that the track density, i.e., track spacing, can be increased which would permit playing time to be increased. It should be noted however, that this scheme cannot readily be used in a system in which a high frequency carrier is read out by a read beam capable of resolving the carrier. This obtains because there will, at times, be a destructive interference between the carrier signals on adjacent tracks and this interference will effectively destroy any output signal. If destructive interference is to be avoided, care must be taken that the carrier signals on adjacent tracks always remain in phase. This phasing relationship is not only very difficult to achieve in practice, but, in the case where use of a frequency modulated carrier is contemplated, it would be impossible since the phase changes continuously in a frequency modulation scheme.